The present invention relates to improving the efficiency and productivity of the filtration of solids from liquid-solid mixtures. More particularly, the invention relates to the filtration of insoluble solids from superphosphoric acid. As used herein the term "superphosphoric acid" refers to an acid having a concentration of P.sub.2 O.sub.5 of between about 68 and 72% by weight.
In the commercial production of phosphoric acid, an acid with a concentration of P.sub.2 O.sub.5 of about 70% by weight is referred to as "superphosphoric acid." Superphosphoric acid is a precurser of other commercial chemical compounds, such as fertilizers. Consequently, in most cases it is necessary that the superphosphoric acid meet certain commercial standards. Because of the method of production of superphosphoric acid, and becuase of its peculiar highly concentrated nature, certain solid impurities or precipitates are characteristically present in the superphosphoric acid when it is produced. These must be removed before the superphosphoric acid is suitable for certain commercial applications. Additionally, the solid impurities or precipitates can cause difficulty in handling the acid after it has been produced.
Superphosphoric acid is generally produced by the two-step evaporation of less concentrated phosphoric acid. The raw material in the production process is phosphate ore, present in which are many of the impurities which later must be filtered out as precipitates in the superphosphoric acid. Sulfuric acid is added to the phosphate ore and produces phosphoric acid with a concentration of about 30% P.sub.2 O.sub.5 by weight. This acid is filtered to remove gross impurities and then evaporated to a concentration of about 54% P.sub.2 O.sub.5 by weight. The 54% acid is then purified either by filtration or by letting insolubles settle out. Finally, the 70% by weight superphosphoric acid is produced by further evaporation of the 54% acid. The nature of the 70% superphosphoric acid is such that materials which are soluble in 54% acid, and thus cannot be filtered out at that concentration, become insoluble in superphosphoric acid and must be filtered out at the higher concentration. Thus, filtration of superphosphoric acid is a necessary step in the production process regardless of the purity of the 54% acid from which it is made.
As is known in the art, filtration of superphosphoric acid presents several problems. The acid is very viscous, highly reactive and impurities are present as relatively small-sized crystals. Furthermore, impurities present in the acid can cause problems in commercial chemical compounds which use the acid as a precursor; for example, liquid fertilizers. As examples of the problems surrounding superphosphoric acid filtration, U.S. Pat. Nos. 4,409,194; 3,632,329; and 3,554,728 all teach methods of removing impurities from either superphosphoric acid itself, or from commercial fertilizers prepared from superphosphoric acid. All of these references, however, teach methods of favorably precipitating solids from either the acid or its by-products; none teaches an improved method of filtering the acid in its initial manufactured state.
One method of filtration of superphosphoric acid is by vacuum filtration such as with a rotary vacuum drum precoat filter. In this system, a precoat of filter aid is applied to the screen surface of a rotating drum, to the interior of which a vacuum is applied. The vacuum draws the superphosphoric acid and its solid impurities onto the surface of the precoat. The nature of the filter aid is such that the superphosphoric acid passes through while the solid impurities remain on the precoat.
In filtration by the rotary vacuum drum precoat filter a problem arises as the acid is filtered; namely, the surface of the precoat becomes coated with the filtered impurities. Without a clean surface of precoat, the efficiency of the filtration system drops rapidly. The solution to this problem is the "doctor" or "doctoring" knife. The doctor knife is set near the rotating drum and serves to scrape off a certain amount of the outermost layer of precoat as the drum rotates past the knife. By removing an outer layer of precoat, the knife also removes the layer of solid impurities built upon the outer surface of the precoat. The result is that a fresh surface of precoat is presented to the acid to be filtered each time the drum rotates past the knife.
As a usual practice, a filtering cycle is begun by building up a desired thickness of filter aid upon the screen surface of the rotating drum of a desired thickness of filter aid. As the acid is filtered, the doctor knife is slowly advanced towards the drum so that it may continuously remove filter aid contaminated with impurities from the drum. It will be seen that the knife will eventually advance enough to remove sufficient filter aid from the drum so that filtration can no longer take place. At this point the filtering system is "recycled" by washing the remaining filter aid upon the drum. It will thus be seen that the rate at which the doctor knife advances through the precoat on the drum is the primary variable in determining the time necessary for one filtration cycle to take place.
As is known in the art, a desirable filter aid should be made up of non-compressible particles, should be of low bulk density, should be porous, and should be chemically inert to the filtrate. When properly chosen, a filter aid is particularly useful in filtering finely divided solids and slimy "floc" type precipitates.
One well known filter aid material is diatomaceous earth. Diatomaceous earth is a porous, mainly silica, material made up of the shells of diatoms, which in turn are a group of algae characterized by their symmetrical shell walls.